The present invention relates to an apparatus for measuring displacement by utilizing a wire.
Conventionally, as an apparatus for measuring a displacement along the stroke of a cylinder, there is known an apparatus that uses an encoder and a wire taken up by a coil spring.
The wire is taken up by a pulley constituting a take up portion. The pulley is energized by the coil spring in the direction in which the wire is taken up. A displacement along the stroke of the cylinder is measured by the direction and amount of rotation of the pulley.
There is, however, a demerit with the use of the conventional displacement measuring apparatus that the size of the pulley around its axis becomes large. Therefore, there has been a danger that the apparatus becomes larger according as the measured distance is increased and it interferes with other units.
Further, there has been such a problem that a coil spring is not durable, i.e., when it is expanded and contracted repeatedly, its elasticity becomes poor resulting in deterioration in the measurement accuracy.
When the measurement is carried out only by the use of an encoder, it has been difficult for a conventional apparatus to secure improved accuracy in measuring the distance.
It is a primary object of the present invention to provide a displacement measuring apparatus smaller in size so that its interference with other units is avoided.
It is another object of the present invention to provide a measuring apparatus of which the measurement accuracy does not deteriorate easily.
A displacement measuring apparatus according to the invention is an apparatus for measuring displacement by utilizing a wire. There is movably disposed a pulley. There is disposed an energizing means such that the pulley is moved under the energizing force. A wire is passed around the pulley. When the wire is let in or let out, the pulley is moved against the energizing force of the energizing means.
A linear sensor may be disposed in the displacement measuring apparatus so that the let out amount or let in amount of the wire is measured by the linear sensor.
An encoder may be disposed in the above displacement measuring apparatus so that the let out amount or let in amount of the wire may also be measured by the encoder.
In the displacement measuring apparatus according to the invention, there is movably disposed a pulley in the body of the apparatus. It is preferred that a plurality of pulleys are used as a unit. There is disposed an energizing means (for example a spring) such that the pulleys are moved under the energizing force. A wire is passed around the pulleys. When the wire is led into or taken out of the body of the apparatus, the pulleys are moved against the energizing force of the energizing means.
Preferably, there is provided a linear sensor in the displacement measuring apparatus such that the let out amount or let in amount of the wire is measured by the linear sensor.
More preferably, there is disposed an encoder in the displacement measuring apparatus provided with the linear sensor so that the let out amount or let in amount of the wire is measured not only by the linear sensor but also by the encoder.
The displacement measuring apparatus configured as described above can be used as a displacement measuring apparatus of a cylinder used in a heavy machine such as a shovel-car.
In the above described case, it is possible that a linear displacement of the pulleys in the body is measured with the linear sensor to obtain the let out amount of the wire as a coarse measurement and the angle of rotation of the encoder is measured as a fine measurement. Thus, it is preferred that the two measurements with the encoder and the linear sensor be combined.
In measuring a linear displacement by means of the linear sensor, it is preferred, in terms of durability and cost, to carry out the measurement utilizing changes in the resistance value with the use of a conducting plastic member.